Shulou(Shulou.com)06/02 Report--

In this issue, the editor will bring you about how to use regular expressions to achieve location matching in javascript. The article is rich in content and analyzes and describes for you from a professional point of view. I hope you can get something after reading this article.

I. Preface

A regular expression is a matching pattern, either a matching character or a matching position.

2. What is location

Position: the position between adjacent characters.

How to match the position

In ES5, there are six anchors: ^, $,\ b,\ B, (? = p), (?! P)

Visual form:

RegExp:/ ^ $\ b\ B (? = a) (?! B) / g

3.1 ^ and $

^ (delimited) matches the beginning, and matches the beginning of a line in a multiline match.

$(dollar sign) matches the end, and matches the end of the line in a multi-line match.

For example, we replace the beginning and end of a string with a # (position can be replaced with a character):

Var result = "hello" .replace (/ ^ | $/ g,'#'); console.log (result); / / "# hello#"

When there is a multi-line matching pattern (with the modifier m), the two are the concept of lines, which we need to note:

Var result = "I\ nlove\ njavascript". Replace (/ ^ | $/ gm,'#'); console.log (result); / / # love#// javascript#

3.2\ b and\ B

\ b is the word boundary, specifically the position between\ w and\ W, including the position between\ w and ^, and the position between\ w and $.

For example, examine\ b in the file name "[JS] Lesson_01.mp4", as follows:

Var result = "[JS] Lesson_01.mp4" .replace (/\ b JS# g,'#'); console.log (result); / / "[# JS#] # Lesson_01#.#mp4#"

First, we know that\ w is an abbreviation for the character group [0-9a-zA-Z], even if it is alphanumeric or any character in the underscore. And\ W is the abbreviation of the character group [^ 0-9a-zA-Z], that is,\ W is any character other than\ w.

Let's take a look at how # came about:

The first, the characters on both sides are "[" and "J", which is the position between\ W and\ w.

The second character on both sides is "S" and "]", that is, the position between\ w and\ W.

Third, the characters on both sides are spaces and "L", that is, the position between\ W and\ w.

Fourth, the characters on both sides are "1" and ".", that is, the position between\ w and\ W.

The fifth, the characters on both sides are "." And "m", which is the position between\ W and\ w.

The sixth, at the end, the preceding character "4" is\ w, that is, the position between\ w and $.

After knowing the concept of\ b, then\ B is easy to understand.\ b is the word boundary and\ B is the non-word boundary.

Var result = "[JS] Lesson_01.mp4" .replace (/\ BJOG,'#'); console.log (result); / / "# [Jacks] # L#e#s#s#o#n#_#0#1.m#p#4"

3.3 (= p) and (?! P)

(? = p) where p is a subpattern, that is, the position before p (the characters after this position should match p)

For example: (? = e), which indicates the position in front of the e character

Var result = "hello". Replace (/ (? = l) / g,'#'); console.log (result); / / "he#l#lo"

And (?! P) is the opposite of (? = p).

Var result = "hello". Replace (/ (?! l) / g,'#'); console.log (result); / / "# h#ell#o#"

Their scientific names are positive lookahead and negative lookahead respectively.

Chinese translation is positive antecedent assertion and negative antecedent assertion respectively.

Versions after ES5 will support positive lookbehind and negative lookbehind.

What exactly is (? IV. Position characteristics

For the understanding of the position, we can accumulate the amount into empty characters.

For example, the "hello" string is equivalent to the following form:

"hello" = "" + "h" + "" + "e" + "" + "l" + "" + l "+"+" o "+"

It is also equivalent to

"hello" = "" + "" + "hello"

Therefore, there is no problem with writing / ^ hello$/ as / ^ hello$$$/,:

Var result = / ^ hello$$$/.test ("hello"); console.log (result); / / true

In other words, the position between characters can be written as multiple.

Note: understanding the position of null characters is a very effective way to understand the position.

V. related cases

5.1 does not match the rules of anything

/. ^ /

The rule requires only one character, but the character is followed by a beginning, and such a string does not exist.

The thousand-minute separator representation of 5.2 numbers

For example, 12345678 becomes 12345678.

Analysis: that is, the corresponding position needs to be replaced with ","

5.2.1 get the last comma

Regular: / (? =\ d {3} $) /

Var result = "12345678" .replace (/ (? =\ d {3} $) / g,',') console.log (result); / / 12345678

Where (? =\ d {3} $) matches the position before\ d {3} $. And\ d {3} $matches the last 3 of the target string as a number.

5.2.2 how many commas do you get?

Because of the location of the comma, the next three numbers are required to appear at least once, that is,\ d {3}.

You can use quantifier +:

Var result = "12345678" .replace (/ (? = (\ d {3}) + $) / g,',') console.log (result); / / 12345678

5.2.3 match the rest of the cases

After writing the rule, you need to use individual cases to verify it, and you will find the problem:

Var result = "123456789" .replace (/? = (\ d {3}) + $) / g,',') console.log (result); / /, 123456789

The above regular simply means to replace the forward number from the end, once it is a multiple of 3, to replace the position in front of the bar with a comma.

It is also required that the matching position cannot be the beginning.

We know that the beginning of the match is to use ^, but not the beginning how to do?

Use (?! ^)

Var regex = / (?! ^) (? = (\ d {3}) + $) / console.log var result = "12345678" .replace (regex,',') console.log (result); / / "12345678" result = "123456789" .replace (regex,','); console.log (result); / / "123456789"

5.2.4 support for other forms

If you want to replace "12345678 123456789" with "12345678 123456789".

At this point, we need to modify the rule, changing the beginning ^ and the end $inside to\ b:

Var string = "12345678 123456789", regex = / (?!\ b) (? = (\ d {3}) +\ b) / gpolivar result = string.replace (regex,',') console.log (result); / / "12345678 123456789"

Among them (?!\ b) how to understand?

The requirement is the current location, but not the position in front of\ b. Actually (?!\ b) means\ B.

So eventually the regular becomes: /\ B (? = (\ d {3}) +\ b) / g.

Visual form:

RegExp:/\ B (? = (\ d {3}) +\ b) / g

5.2.5 currency formatting

A common application of thousand symbol representation is currency formatting.

Put this string:

Format:

$1888.00

Achieve:

Function format (num) {return num.toFixed (2). Replace (/\ B (? = (\ d {3}) +\ b) / g, ","). Replace (/ ^ /, "$");}; console.log (format (1888)); / / "$1888.00"

5.3 issues with verifying passwords

The password is 6-12 in length and consists of numbers, lowercase letters and uppercase letters, but must include at least 2 characters.

It is easier to judge by writing multiple regularities, but it is more difficult to write one regular.

Let's see if we have a deep understanding of location.

5.3.1 Simplification

Without considering the condition that "must contain at least 2 characters" for the time being, it is easy to write:

Var regex = / ^ [0-9A-Za-z] {6Jing 12} $/

5.3.2 determine whether a certain character is included

Hypothetically, we are required to include numbers, how to do it? At this point, we can use (? =. * [0-9]) to implement it.

Regularization becomes:

Var regex = / (? =. * [0-9]) ^ [0-9A-Za-z] {6pm 12} $/

5.3.3 contains both specific characters

For example, if you include both numbers and lowercase letters, you can use (? =. * [0-9]) (? =. * [aMuz]).

Regularization becomes:

Var regex = / (? =. * [0-9]) (? =. * [Amurz]) ^ [0-9A-Za-z] {6jue 12} $/

5.3.3 concrete implementation

Change the original question into the following situations:

1. Contain both numbers and lowercase letters

2. Contain both numbers and uppercase letters

3. Contain both lowercase and uppercase letters

4. Contains both numbers, lowercase letters and uppercase letters.

In the above 4 cases, it is the relationship of or (in fact, Article 4 may not be used).

Final answer:

Var regex = / ((? =. * [0-9]) (? =. * [Amurz]) | (? =. * [0-9]) (? =. * [Amurz])) | (? =. * [AZ]) ^ [0-9A-Za-z] {6 9A-Za-z 12} $/; console.log (regex.test ("1234567")); / / false is all digital console.log (regex.test ("abcdef")) / / false is all lowercase letters console.log (regex.test ("ABCDEFGH")); / / false is all uppercase letters console.log (regex.test ("ab23C")); / / false is less than 6 digits of console.log (regex.test ("ABCDEF234")); / / true uppercase letters and numbers console.log (regex.test ("abcdEF234")); / / true has all three

Visual form:

RegExp:/ ((?) =. * [0-9]) (? =. * [a murz]) | (? =. * [0-9]) (? =. * [Amurz]) | (? =. * [Amurz]) (? =. * [AZ]) ^ [0-9A-Za-z] {6pm 12} $/

Analysis:

The above regular looks so complicated that you only need to understand the second step, / (? =. * [0-9]) ^ [0-9A-Za-z] {6jue 12} $/

For this rule, we need to understand (? =. * [0-9]) ^ this

Separately, they are (? =. * [0-9]) and ^.

Indicates that there is a position before the beginning (of course, the beginning, that is, the same position, think of the previous empty character analogy).

(? =. * [0-9]) indicates that the characters after that position match.

. * [0-9], that is, any number of arbitrary characters followed by a number.

Translated into vernacular, that is, the following characters, must contain a number.

5.3.4 another solution

"contains at least two characters" means that it cannot be all numbers, all lowercase letters, and not all uppercase letters.

So what should we do if we ask that "not all numbers can be made up of numbers"? (?! P) come on!

Corresponding regularity:

Var regex = / (?! ^ [0-9] {6pm 12} $) ^ [0-9A-Za-z] {6jue 12} $/

What about all three "can't"?

The final answer is:

Var regex = / (?! ^ [0-9] {6mague 12} $) (?! ^ [Amurz] {6mague 12} $) (?! ^ [Amurz] {6Mae 12} $) ^ [0-9A-Za-z] {6Mae 12} $/; console.log (regex.test ("1234567")); / / false is all numeric console.log (regex.test ("abcdef")); / / false is all lowercase letters console.log (regex.test ("ABCDEFGH")). / / false is all uppercase console.log (regex.test ("ab23C")); / / false is less than 6 digits console.log (regex.test ("ABCDEF234")); / / true uppercase letters and numbers console.log (regex.test ("abcdEF234")); / / true has all three

Visual form:

RegExp:/ (?! ^ [0-9] {6jue 12} $) (?! ^ [a murz] {6je 12} $) (?! ^ [Amurz] {6cr 12} $) ^ [0-9A-Za-z] {6jue 12} $/

The above is the editor for you to share how to use regular expressions in javascript to achieve location matching, if you happen to have similar doubts, you might as well refer to the above analysis to understand. If you want to know more about it, you are welcome to follow the industry information channel.

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